In the same plot, below the purple curve, the \(Z^{\prime }\)  can be cosmologically long lived against decaying into neutrinos and could be a dark matter candidateHere we have assumed the lightest neutrino is massless.
If the lightest neutrino is massive, the region for \(Z^{\prime }\)  to be cosmological stable will be wider.
We do not address the issue of how the \(Z^{\prime }\)  achieves it relic density. However, in that context it is worth noting that the coupling constants we consider are so small that the freeze in process only results in a very
small relic density..
There are proposed experiments to detect directly this type of dark matter by absorption using special materials [1]}.
